Have a look at the wiring to the pressure solenoid on the power steering pump. The ZG had speed proportional power steering, but many garages used to cut the wiring to the solenoid as a kind of fix for their tendency to drift down to the left on our narrow roads with steep cambers. This worked to a certain extent in that it disabled the systems ability to reduce assistance at the central position, so the driver felt he wasn't having to steer off centre all the time. BTW, I don't think diesels had this so disregard if yours is diesel.